Graph Powers of Groups II: The RA Matrix
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arXiv
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| Format: | Preprint |
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2025
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| _version_ | 1866917005337034752 |
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| author | Cunningham, Gabe Minevich, Igor |
| author_facet | Cunningham, Gabe Minevich, Igor |
| contents | For a graph $Γ$ and group $G$, $G^Γ$ is the subgroup of $G^{|Γ|}$ generated by elements with $g$ in the coordinates corresponding to $v$ and its neighbors in $Γ$. There is a natural epimorphism $G^Γ\to (G/[G,G])^Γ$ with kernel $[G,G]^n \cap G^Γ$. When $[G,G]^n \leq G^Γ$, the structure of $G^Γ$ is easily described from $(G/[G,G])^Γ$. Fixing $Γ$, if $[G,G]^{|Γ|} \leq G^Γ$ for all $G$, we say that $Γ$ is RA (reducible to abelian). We showed in [2] that wide classes of graphs are RA, including graphs of girth 5 or more. The key tool is the RA matrix $C_Γ$, and we showed that $Γ$ is RA if and only if the row space $Row(C_Γ) = \mathbb Z^{|Γ|}$.
Here, we study the possibilities for the elementary divisors of $C_Γ$; the more nontrivial elementary divisors we get, the further $Γ$ is from being RA (and the harder $G^Γ$ is to describe). We show that while many graphs, including those of girth 4, cartesian products, and most tensor products have at most one nontrivial elementary divisor, one can construct a graph of girth 3 with any prescribed set of elementary divisors and $\mathbb Z$-nullity. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2510_10314 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Graph Powers of Groups II: The RA Matrix Cunningham, Gabe Minevich, Igor Combinatorics Group Theory For a graph $Γ$ and group $G$, $G^Γ$ is the subgroup of $G^{|Γ|}$ generated by elements with $g$ in the coordinates corresponding to $v$ and its neighbors in $Γ$. There is a natural epimorphism $G^Γ\to (G/[G,G])^Γ$ with kernel $[G,G]^n \cap G^Γ$. When $[G,G]^n \leq G^Γ$, the structure of $G^Γ$ is easily described from $(G/[G,G])^Γ$. Fixing $Γ$, if $[G,G]^{|Γ|} \leq G^Γ$ for all $G$, we say that $Γ$ is RA (reducible to abelian). We showed in [2] that wide classes of graphs are RA, including graphs of girth 5 or more. The key tool is the RA matrix $C_Γ$, and we showed that $Γ$ is RA if and only if the row space $Row(C_Γ) = \mathbb Z^{|Γ|}$. Here, we study the possibilities for the elementary divisors of $C_Γ$; the more nontrivial elementary divisors we get, the further $Γ$ is from being RA (and the harder $G^Γ$ is to describe). We show that while many graphs, including those of girth 4, cartesian products, and most tensor products have at most one nontrivial elementary divisor, one can construct a graph of girth 3 with any prescribed set of elementary divisors and $\mathbb Z$-nullity. |
| title | Graph Powers of Groups II: The RA Matrix |
| topic | Combinatorics Group Theory |
| url | https://arxiv.org/abs/2510.10314 |